Acrylonitrile is an important commodity chemical used mainly as monomer for the manufacture of a wide variety of polymeric materials such as polymers for acrylic fibers used in textiles, and in resins such as ABS and SAN resins. Worldwide, acrylonitrile is produced in amounts exceeding four million metric tons per year. The most commonly used process for manufacturing acrylonitrile is to react a suitable hydrocarbon such as propylene or propane in an ammoxidation reactor in the presence of ammonia using air or other source of molecular oxygen as an oxidant. Such oxidation reactions, also called ammoxidation reactions, typically use a solid-particulate heterogeneous catalyst in a fluidized catalyst bed to catalyze the ammoxidation reaction and provide the desired acrylonitrile in acceptable conversion and yield. In addition to producing acrylonitrile, such ammoxidation reactions also generally produce acetonitrile, hydrogen cyanide (HCN) and other valuable co-products. Acetonitrile is used, for example, as a solvent. Process for the catalytic ammoxidation of a hydrocarbon feed to acrylonitrile are disclosed, for example, in U.S. Pat. Nos. 5,093,299; 4,863,891; 4,767,878; 4,503,001, all of which are incorporated herein by reference.
The processes widely used in commercial practice for recovering the products of such hydrocarbon ammoxidation, such as the ammoxidation of propylene, generally comprise the steps of: a) contacting the effluent from an ammoxidation reactor in a quench tower with an aqueous quench liquid to cool the gaseous effluent; b) contacting the quenched effluent with water in an absorber, forming an aqueous solution comprising the ammoxidation products; c) subjecting the aqueous solution to a water extractive distillation, and d) removing a first overhead vapor stream comprising acrylonitrile and some water from the top of the column, and collecting a liquid waste stream containing water and contaminants from the bottom of the column. Further purification of the acrylonitrile may be accomplished by passing the overhead vapor stream to a second distillation column to remove at least some impurities from the crude acrylonitrile, and further distilling the partially purified acrylonitrile. The effluent from the ammoxidation reactor generally contains a certain amount of ammonia. Therefore, the quench fluid used in the quench tower may also contain a strong mineral acid, such as sulfuric acid, to react with and thereby form the water soluble salt of ammonia, such as ammonium sulfate. The used or spent quench fluid containing the ammonium sulfate and other components is typically treated or disposed of in an environmentally safe manner.
Crude acetonitrile can be obtained as a bottoms stream from a column used to perform the above-mentioned extractive distillation and can then be subjected to further purification procedures such as, for example, the purification procedures disclosed in U.S. Pat. No. 6,204,407.
It would be desirable to be able to adjust the relative amounts of acrylonitrile and acetonitrile produced by the ammoxidation of a hydrocarbon feed material such as propylene or propane because, at times, it is desirable to have additional acetonitrile available to meet changing or increased market demands. Additionally, it would be desirable to produce a spent or used quench liquid that can be recycled within the process for producing acrylonitrile rather than treating for disposal. The present invention provides such processes.
The present invention relates to a process for the ammoxidation of carboxylic acids, especially a mixture of carboxylic acids, to a nitrile or mixture of nitrites. The present invention also relates to increasing the yield and, preferably, the ratio of co-product acetonitrile to acrylonitrile produced during the ammoxidation of a hydrocarbon such as propylene or propane to acrylonitrile.
The present invention also relates to a process which increases the yield of co-product acetonitrile during the manufacture of acrylonitrile by the ammoxidation of a hydrocarbon feed material such as propylene or propane while saving on the raw material costs associated with the increase in co-product yields. The relative amounts of acetonitrile can be controlled by the process of this invention by adjusting the amount of carboxylic acid added to the ammoxidation reaction.
It has been found, unexpectedly, that the use of carboxylic acids, especially crude carboxylic acid mixtures, can attain a desirable increase in the production of acetonitrile during the production of acrylonitrile.
The present invention also relates to a process for recycling spent quench tower quench liquid to the ammoxidation reactor.